1. Field of the Invention
The present invention relates to an image recording apparatus which applies heat and pressure to a predetermined print medium to heat fuse a toner image onto the print medium.
2. Description of Related Art
Conventionally, an electrophotographic recording type image recording apparatus for forming an image by heat fusing a toner image onto a predetermined print medium has been known. Such an image recording apparatus includes processes of charging and exposing a photo conductor, developing an electrostatic latent image formed on the photo conductor by a toner, transferring of thus obtained toner image onto the print medium and fusing the toner image onto the print medium, resulting in a formation of an image.
In this type of image recording apparatus, a constant control of a temperature of a fusing roller when a printing paper as the print medium passes therethrough improves a prevention of a fusing nonuniformity. However, in this type of image recording apparatus, a temperature of an area of the fusing roller where the printing paper does not pass therethrough becomes higher than a temperature of an area of the fusing roller where the printing paper passes therethrough in a case where a width of the printing paper is shorter than a width of the fusing roller, resulting in the fusing nonuniformity at end portions of the printing paper.
To resolve the above problems, techniques to prevent such fusing nonuniformity have been proposed (for example, see Japanese Patent Laying-Open Publication No. 2001-20978).
Specifically, Japanese Patent Laying-Open Publication No. 2001-201978 discloses a fusing device including a fusing roller, a heating roller having a heat source therein, an endless fusing belt stretched between these rollers, a pressurizing roller disposed opposing to the fusing roller through the belt, in which a toner image on the printing paper is conveyed to a space between the pressurizing roller and the fusing belt to achieve fusing of the toner image. More specifically, the fusing device includes a first heater having a heat distribution corresponding to a width of a small sized printing paper and a second heater having a heat distribution corresponding to both end portions other than the heat distribution of the first heater, in which only the first heater is used when a size of the printing paper is small whereas the first and second heaters are used when the size of the printing paper is large, namely, a use of the heaters having individual heat distribution is switched over according to the sizes of the printing papers.
However, in the conventional fusing device as stated in the above Japanese Patent Laying-Open Publication No. 2001-201978, if fusing is attempted to be performed onto printing papers of various widths such as a range of sizes of A5 lengthwise size to A3 lengthwise extended size, only a switching of a use of two heaters between a large size and a small size often raises a problem of an occurrence of the fusing nonuniformity, since there occurs a large temperature difference between temperatures of end portions of the printing paper and a temperature of a center portion of the printing paper in terms of the printing papers belonging to a range of middle width of between a maximum width and a minimum width such as A4 lengthwise size and B4 lengthwise size, thereby inducing an occurrence of the fusing nonuniformity.
Also, in the conventional fusing device, there is the temperature difference between the end portions and the central portion of the printing paper due to a thickness of the printing paper and/or a conveyance speed of the printing paper upon fusing even in the case of the printing paper of the same size. If the temperature difference becomes large, there arises a problem that the fusing nonuniformity still occurs.
The present invention was made in consideration of the above stated current condition. The present invention aims to provide an image recording apparatus capable of minimizing the temperature difference caused by differences of conditions such as a size and/or a conveyance speed between the end portions and the central portion of the printing paper in order to prevent the fusing nonuniformity, resulting in achieving a good printing quality.